Align two faces without math
In the 3D viewports menu bar, enable snapping by click the Magnet Icon. Next to the icon, in the snapping to dropdown menu. Select Face, since you want to snap to a face and choose Active.



By moving an object in object mode you can snap its origin to other faces. Go into edit mode &#8633; Tab, go into face mode 3. Select the face which you want to snap (it gets highlighted as the active face) and add all other faces which you want to move to the selection. (Box Select or A)

Press G to move the geometry and by hovering over the face of the other object with mouse, the face snaps to it. Press X during the G operation to constrain the movement to the X-Axis.





If you want to automatically modify the orientation while snapping to an oblique face, check Align Rotation to Target in the snapping settings.



The position the objects center at the face you wish to snap and snap it in object mode.